Provided by: netpbm_11.01.00-2build1_amd64 
NAME
pngtopam - convert a PNG image into a Netpbm image
SYNOPSIS
pngtopam [-verbose] [-alphapam | -alpha | -mix] [-background=color] [-gamma=value]
[-text=filename] [-time] [-byrow] [pngfile]
Minimum unique abbreviation of option is acceptable. You may use double hyphens instead
of single hyphen to denote options. You may use white space in place of the equals sign
to separate an option name from its value.
DESCRIPTION
This program is part of Netpbm(1).
pngtopam reads a PNG image (Portable Network Graphics) as input and produces a Netpbm
image as output. The type of the output file depends on the input file - if it's black &
white, pngtopam creates a PBM file. If it's grayscale, pngtopam creates a PGM file.
Otherwise, it creates a PPM file. Except that with the -alphapam option, it always
creates a PAM file. That file has tuple type GRAYSCALE_ALPHA or RGB_ALPHA depending on
whether the input has color or not.
To convert in the other direction, use pamtopng or pnmtopng. The former is the more
modern of the two and can recognize transparency information in a PAM file, as you might
generate with pngtopam -alphapam. It has existed only since June 2015. The latter has
more features, but probably not ones that matter in the modern world.
OPTIONS
In addition to the options common to all programs based on libnetpbm (most notably -quiet,
see
Common Options ⟨index.html#commonoptions⟩ ), pngtopam recognizes the following command
line options:
-verbose
Display various information about the input PNG image and the conversion process.
If you want even more information about the PNG image, use pngcheck (not part of
Netpbm).
-alphapam
Produce a single output image containing the main image (foreground) and the
transparency channel or transparency mask. This image is in the PAM format with
tuple type of either GRAYSCALE_ALPHA (which has a depth of 2 channels) or RGB_ALPHA
(which has a depth of 4 channels).
You can specify only one of -alphapam, -alpha, and -mix. With none of them,
pngtopam produces an image of the foreground of the input image and discards
transparency information.
This option was new in Netpbm 10.44 (September 2008).
-alpha Output the transparency channel or transparency mask of the image. The result is
either a PBM file or a PGM file, depending on whether different levels of
transparency appear.
pngtopam discards the main image (the foreground).
You can specify only one of -alphapam, -alpha, and -mix. With none of them,
pngtopam produces an image of the foreground of the input image and discards
transparency information.
-mix Compose the image with the transparency or transparency mask against a background.
The background color is determined by the bKGD chunk in the PNG, except that you
can override it with -background. If the PNG has no bKGD chunk and you don't
specify -background, the background color is white.
You can specify only one of -alphapam, -alpha, and -mix. With none of them,
pngtopam produces an image of the foreground of the input image and discards
transparency information.
-background=color
This option specifies the background color with which to mix the image when you
specify -mix.
color is as described for the argument of the pnm_parsecolor() library routine
⟨libnetpbm_image.html#colorname⟩ .
Examples:
• -background=rgb:01/ff/80
• -background=rgbi:1/255/128
If you don't specify -background, the background color is what is specified in the
PNG image, and if the PNG doesn't specify anything, white.
You cannot specify -background unless you also specify -mix. Before Netpbm 10.27
(March 2005), you could specify -background without -mix and it was just ignored.
(This caused a usability problem).
-gamma=value
This option causes pngtopam to respect the image gamma information in the PNG file
(from the gAMA chunk). Probably by historical accident, pngtopam ignores that
information by default, assuming the image uses the same gamma transformation as a
Netpbm image, so the output image has different colors than the PNG file actually
represents if the PNG doesn't actually do that. (However, it is rare for a PNG
file to use a gamma transformation different from what the Netpbm formats specify,
or if it does, to specify with a gAMA chuck what that is).
But when you do specify -gamma, you get a rather strange additional function,
probably a historical mistake: pngtopam incorporates the specified screen gamma
value into the output pixels, so that the samples in the Netpbm output deviate from
the Netpbm format specifications and are appropriate raw intensity values to send
to the display. This function essentially just exercises the ability of the PNG
library to make gamma corrections to the pixels as it reads them from the PNG file
to produce values appropriate for sending to a certain display in certain viewing
conditions. It's a strange function because it has nothing to do with PNG and
because in Netpbm, the normal way to make gamma corrections appropriate for sending
to a ceratin display in certain viewing conditions is with the program pngtopam,
applied to the normal output of pngtopam.
If you specify -gamma, but the PNG image does not specify what gamma transformation
it uses (there is no gAMA chunk), pngtopam assumes a simple power transformation
with an image gamma of 1.0. That is probably not not the actual image gamma; it is
much more likely to be .45.
Because the gammas of uncompensated monitors are around 2.6, which results in an
image-gamma of 0.45, some typical situations are: when the image-gamma is 0.45 (use
-verbose to check) and the picture is too light, your system is gamma-corrected, so
convert with "-gamma 1.0". When no gAMA chunk is present or the image-gamma is
1.0, use 2.2 to make the picture lighter and 0.45 to make the picture darker.
One oddity to be aware of when using -gamma on an image with transparency: The PNG
image specifies that a certain color is transparent, i.e. every pixel in the image
of that color is transparent. But pngtopam interprets this as applying to the
gamma-corrected space, and there may be less precision in that space than in the
original, which means multiple uncorrected colors map to the same corrected color.
So imagine that the image contains 3 shades of white (gray) and specifies that one
of them is transparent. After gamma correction, those three shades are
indistinguishable, so pngtopam considers pixels of all three shades to be
transparent.
-text=file
Writes the tEXt and zTXt chunks to a file, in a format as described in the pnmtopng
user manual. These chunks contain text comments or annotations.
-time Prints the tIME chunk to stderr.
-byrow This option can make pngtopam run faster or in environments where it would
otherwise fail.
pngtopam has two ways to do the conversion from PNG to PAM, using respectively two
facilities of the PNG library:
Whole Image
Decode the entire image into memory at once, using png_read_image(), then convert
to PAM and output row by row.
Row By Row
Read, convert, and output one row at a time using png_read_row().
Whole Image is generally preferable because the PNG library does more of the work,
which means it understands more of the PNG format possibilities now and in the
future. Also, if the PNG is interlaced, pngtopam does not know how to assemble the
rows in the right order.
Row By Row uses far less memory, which means with large images, it can run in
environments where Whole Image cannot and may also run faster. And because Netpbm
code does more of the work, it's possible that it can be more flexible or at least
give better diagnostic information if there's something wrong with the PNG.
The Netpbm native code may do something correctly that the PNG library does
incorrectly, or vice versa.
In Netpbm, we stress function over performance, so by default pngtopam uses Whole
Image. You can select Row By Row with -byrow if you want the speed or resource
requirement improvement.
-byrow was new in Netpbm 10.54 (March 2011).
-orientraw
A TIFF stream contains raster data which can be arranged in the stream various
ways. Most commonly, it is arranged by rows, with the top row first, and the
pixels left to right within each row, but many other orientations are possible.
The common orientation is the same on the Netpbm formats use, so tifftopnm can do
its jobs quite efficiently when the TIFF raster is oriented that way.
But if the TIFF raster is oriented any other way, it can take a considerable amount
of processing for tifftopnm to convert it to Netpbm format.
SEE ALSO
pamtopng(1), pnmtopng(1), pngtopnm(1), ptot, pnmgamma(1), pnm(1)
For information on the PNG format, see http://schaik.com/png ⟨http://schaik.com/png⟩ .
NOTE
A PNG image contains a lot of information that can't be represented in Netpbm formats.
Therefore, you lose information when you convert to another format with "pngtopam |
pnmtoxxx". If there is a specialized converter that converts directly to the other
format, e.g. ptot to convert from PNG to TIFF, you'll get better results using that.
LIMITATIONS
There could be an option to include PNG comment chunks in the output image as PNM comments
instead of putting them in a separate file.
The program could be much faster, with a bit of code optimizing. As with any Netpbm
program, speed always takes a back seat to quick present and future development.
HISTORY
pngtopam was new in Netpbm 10.44, as a replacement for pngtopnm. The main improvement
over pngtopnm was that it could generate a PAM image with a transparency channel, whereas
with pngtopnm, you would have to extract the transparency channel as a separate file, in a
separate run.
pngtopnm was new in Netpbm 8.1 (March 2000), the first big change to the package in
Netpbm's renaissance. It and pnmtopng were simply copied from the pnmtopng package" (1)
by Greg Roelofs. Those were based on simpler reference applications by Alexander Lehmann
<alex@hal.rhein-main.de> and Willem van Schaik <willem@schaik.com> and distributed with
their PNG library.
Nearly all of the code has changed since it was copied from the pnmtopng package, most of
it just to improve maintainability.
AUTHORS
Copyright (C) 1995-1997 by Alexander Lehmann and Willem van Schaik.
DOCUMENT SOURCE
This manual page was generated by the Netpbm tool 'makeman' from HTML source. The master
documentation is at
http://netpbm.sourceforge.net/doc/pngtopam.html